1. Field of the Invention
This invention relates to a method for processing topaz by the combined use of high energy neutron radiation and electron bombardment to produce a novel and superior product and to the novel product so produced by this unique processing method.
More particularly, this invention relates to the processing of colorless or pale colored topaz to produce a new product of a different color, particularly of the moderately deeper blue, which I describe as "American Blue" which color is of better quality and greater value than the original topaz gemstone. A slightly modified method may be used to produce a lighter blue color than the "American Blue" which I describe as "Super Sky Blue".
2. Description of the Prior Art
While it is well known to those skilled in the nuclear art that the bombardment of various gems, glasses and plastics by sub-atomic particles often produces changes in color as well as in other properties of these materials, neither the reason nor the mechanism of the process are comprehensively understood. Except in a few well known cases, the results of such subatomic particle bombardment are neither predictable nor obvious for any given material or type of radiation.
For example, it is known in the prior art that ordinary glass loses its transparency when subjected to gamma radiation. Pyrex, a type of glass, becomes amber and quartz becomes purple. Without special treatment, these colors gradually disappear if the glass is allowed to stand for a prolonged period or is subject to the action of intense light rays. This particular property has been used to detect radiation by noting the darkening of specially treated glass when exposed to further radiation as described by McAlpine and Rinehart in U.S. Pat. No. 2,782,319. On the other hand, various types of glasses have been developed also which resist the color changing effect of radiation such as disclosed in U.S. Pat. No. 2,747,105 which was issued to Fitzgerald, et al.
Likewise, the color and transparency of plastics have been known to be effected by nuclear irradiation as detailed in U.S. Pat. No. 2,855,517 issued to Rainer, et al.
The phenomenon is frequently described as being caused by the displacement of electrons from one part of the material to another resulting in the entrapment of some of the electrons in the color or "F" center of the material thereby changing its isotropy and consequently its color. (Stephenson, "Introduction to Nuclear Engineering," pp. 222, 256, and 350).
In the instant invention, I have discovered that by taking topaz gemstones which are colorless or of certain given colors and by properly subjecting them to the combined two step action of neutron bombardment, and then by electron bombardment, that I am able to permanently and completely change their color without impairing their other qualities.
Further, I have discovered also that by taking topaz gemstones which are colorless or of certain given colors and subjecting them first to high energy neutron bombardment and then bombarding them with high energy electrons I can produce topaz gemstones of a highly desirable, predictable, and permanent color.
As is well known, Topaz is a natural aluminum fluorisilicate which can be written in the formula, [Al.sub.2 F.sub.2 SiO.sub.4 ]. It commonly occurs in high temperature veins probably formed in igneous intrusions in the presence of fluoride and water vapor. As a result, some of the fluoride is usually replaced by hydroxyl, giving it the variable composition, [Al.sub.2 (F,OH).sub.2 SiO.sub.4.]
Topaz occurs naturally in a wide range of colors (blue, yellow, green, orange, violet, pink and the highly desirable gold to sherry), but it is most commonly colorless.
The crystaline structure of topaz was determined independently by Pauling (1928) and by Alston and West (1928). It was described as belonging to the orthorhombic centrosymmetric space group pbnm and the structure has been successfully refined in that space group (Ribbe and Gibbs, 1971).
An important optical property or, more generally many gemstones including topaz is that known as dichroism of pleochroism. A stone possessing this property, when observed in different directions will show different colors or shades of color which may resemble each other more or less closely, or may differ considerably. The pleochroism of natural topaz is distinct but not strong.
The production of blue topaz from colorless stones by irradiation was first reported by F. H. Pough in 1957 as one of a large number of color changes observed in a variety of materials subjected to radiation treatment. The process was rediscovered and reported by Nassau in 1974 and by Nassau and Prescott in 1975.
Many people in the gemstone business believed the reporting of irradiation treatment to produce blue topaz provided an explanation for the large increase in the quantity of blue topaz available at that time. During the last ten years, more than a million carats of blue topaz have entered the world market.
Several types of irradiation can be used to alter the color of topaz including X and gamma rays, high energy charged particles, and neutrons. To date, however, the only generally useful forms of ionizing radiation include gamma rays from Co-60, high energy electrons from accelerators, and neutrons from nuclear reactors.
One of the principal qualities which determines the commercial value of a topaz stone is its color. Naturally occurring blue topaz stones are pale in color and their value in the market is therefore limited.
Only a small percentage of the topaz stones which have been irradiated by electrons will be a sufficiently intense blue color to be marketable. Therefore, the cost of producing the stones that will eventually be sold is increased by the cost for producing stones that are not salable.
When colorless or pale colored topaz stones are irradiated with neutrons, the pleochroism which results in different colors being seen when looking along different axes frequently results in an undesirable gray or "inky" appearance to many of the stones. The value of these stones is much less than the top quality blue gemstones.
When a colorless or pale-colored topaz is exposed to gamma rays, several colors of the yellow to reddish-brown to brown tones develop in some of the stones at relatively low radiation doses, that is, doses of less than one megarad. As the dose is increased, a green-yellow to green blue color will develop in some stones, depending on the nature of the particular stone. If these particular stones are subsequently heat treated, a blue color can be brought out in the stone.
When by the nature of some stones an intense blue color is produced, it is generally a gray or "steely" blue.
In order for electrons to penetrate a significant thickness of topaz, energies in the range of 10 to 20 million electron-volts are required. To obtain these energies, it is necessary to accelerate the electrons in any one of a variety of machines including linear accelerators, Van de Graaff generators, and betatrons. A significant advantage in the use of electron accelerators is that the dose rates available are much higher than with gamma ray sources, enabling sufficient exposures to be given to the stones in only a few hours instead of several months. Another advantage is that the gray blue color produced by gamma rays and neutrons does not generally occur. Instead, in those stones which possess the needed natural property to turn blue, a very clear "sky" blue color is produced.
Neutrons can be produced by a variety of means, but the most practical method to generate the neutron intensities required to color meaningful quantities of topaz in a reasonable time is to use a nuclear reactor.
In general, all colorless or pale colored topaz will turn a blue color if given sufficient exposure to high energy neutrons. The color can be made darker than most intense color from either gamma rays or electrons. The pleochroism of topaz is very evident in neutron irradiated stones with the color axis appearing dark blue, light blue, and gray. This property results in a wide range of colors depending on the orientation of the crystalline axis with respect to the stone and the type of cutting used relative to the particular stone.